Firestorm stores usernames and passwords in an encrypted credentials file inside your user settings folder. This file can only be used on one computer, it can't be decoded on a different computer.
If you don't want the viewer to store passwords:
Firestorm may complain that it cannot decode the credentials when you launch the viewer. This can be due to actual corruption in the file or because the file came from a different computer. In some cases you can simply re-enter your credentials and the file will self-repair. If it warns you on each login, it will be necessary to delete the credentials file.
See this page for documentation on Firestorm 4.7.1 (45325) and earlier.